This invention relates to a diurea grease composition. More particularly, it relates to a diurea grease composition which undergoes little changes in consistency after use at higher temperatures for a prolonged period of time and small oil separation at higher temperatures and which is also superior in various other properties.
Up to now, metal soaps are predominantly employed as the grease thickeners. However, the lithium soap grease, known as the "universal grease", has a dropping point of the order of 200.degree. C. or thereabouts and cannot be used at higher temperatures above 150.degree. C.
On the other hand, various complex soaps, sodium terephthalamate, bentone or organic thickeners, such as indanthrene or urea have been proposed as high temperature long life grease thickeners. However, these thickeners suffer from certain demerits For example, a calcium complex type thickener tends to be hardened markedly with lapse of time. Sodium terephthalamate in general undergoes syneresis and oil separation considerably, while it tends to be deteriorated due to oxidation by reason of intramolecular metal atoms in the thickener. Bentone is inferior in lubricating properties at higher temperatures on prolonged usage, while indanthrene has a bad hue and is expensive.
On the other hand, urea greases, such as diurea grease or tetraurea greases having modified terminal groups, have many advantageous points as compared with the above mentioned greases. However, the tetraurea grease tends to be hardened or increased in consistency when exposed to higher temperatures for prolonged time period, while it may be disadvantageously hardened or softened depending on different shearing speeds given to the grease. The known diurea greases having only terminal alkyl groups have a low dropping point and undergoes considerable oil separation at higher temperatures so that they cannot be used for prolonged time at higher temperatures. The diurea greases having only aromatic hydrocarbon terminal groups are at most equivalent or even inferior to the diurea greases having the terminal alkyl groups with respect to the properties as the thickener.
The present inventors have conducted researches towards overcoming the above mentioned drawbacks of the urea greases and have found that, while the diurea compound has highly desirable properties as the thickener, the terminal groups of the diurea compound plays an extremely important role.
Specifically, it has been found that the diurea compound in which C-6 to C-12 cyclohexyl groups or its derivatives and C-8 to C-20 alkyl groups exist on either one of the terminal ends of the diurea compounds and in which the contents of the cyclohexyl groups or its derivatives are 20 to 90 mol percent based on the total amount of the two groups, exhibits extremely desirable properties as the grease thickener. This diurea compound has been applied for patent in our Japanese Patent Publication No. 11156/1980.
Ths diurea grease disclosed in the Japanese Patent Publication No. 11156/1980 has many excellent properties, such as
i) little changes in consistency after prolonged use at higher temperatures; PA1 ii) superior mechanical stability under a wide range of shearing speeds; PA1 iii) small oil separation at higher temperatures; PA1 iv) superior water resistance; and PA1 v) improved thickening properties.
A diurea grease composition exhibiting extremely low hardening properties with lapse of time besides the above mentioned desirable properties has also been found and applied for patent in Japanese Patent Laid-open Application No. 250097/1987.
Also an urea grease containing two diurea compounds at predetermined ratios which are sandwitched between the terminal group and both urea groups and different with respect to the hydrocarbon groups, is disclosed in our Japanese Patent Application No. 139696/1989.
As a result of our further researches, the present inventors have found that a grease containing a diurea compound obtained by reacting an amine with a mixed system of two or more different diisocyanates exhibits superior properties. This finding has led to fulfilment of the present invention.